ORIGINAL PAPER

Biodegradable Poly(butylene succinate) and Poly(butyleneadipate-co-terephthalate) Blends: Reactive Extrusionand Performance Evaluation

Rajendran Muthuraj • Manjusri Misra •

Amar Kumar Mohanty

� Springer Science+Business Media New York 2014

Abstract Two biodegradable polyesters, poly(butylene

adipate-co-terephthalate) (PBAT) and poly(butylene suc-

cinate) (PBS) were melt-compounded in a twin screw

extruder to fabricate a novel PBS/PBAT blend. The com-

patibility of the blend was attributed to the transesterifi-

cation reaction that was confirmed by Fourier transform

infrared spectroscopy. The Gibbs free energy equation was

applied to explain the miscibility of the resulting blend.

Dynamic mechanical analysis of the blends exhibits an

intermediate tand peak compared to the individual com-

ponents which suggests that the blend achieved compati-

bility. One of the key findings is that the tensile strength of

the optimized blend is higher than each of the blended

partner. Rheological properties revealed a strong shear-

thinning tendency of the blend by the addition of PBAT

into PBS. The phase morphology of the blends was

observed through scanning electron microscopy, which

revealed that phase separation occurred in the blends. The

spherulite growth in the blends was highly influenced by

the crystallization temperature and composition. In addi-

tion, the presence of a dispersed amorphous phase was

found to be a hindrance to the spherulite growth, which was

confirmed by polarizing optical microscopy. Furthermore,

the increased crystallization ability of PBAT in the blend

systems gives the blend a balanced thermal resistance

property.

Keywords Biodegradable polyester � Transesterification �Tensile strength � Morphology

Introduction

The development of biodegradable material as a potential

substitute for non-biodegradable material is an emerging

field of research and development. In recent years, different

types of biodegradable polymers have received an immense

amount of attention for developing various new materials

and to reduce environmental concerns [1]. Some biode-

gradable polymers are commercially available in the market,

such as poly(butylene adipate-co-terephthalate) (PBAT),

polyhydroxyalkanoates (PHAs), polycaprolactone (PCL),

poly(propylene carbonate) (PPC), poly(butylene succinate)

(PBS), poly(lactic acid) (PLA), and thermoplastic starch [2–

5]. Biodegradable polymers are not currently widely used

due to some limitations such as their cost, mechanical

properties, and thermal stability.

Researchers have been trying to address these issues by

utilizing blending techniques to obtain biodegradable blends

with tailored properties. Compared to other methods, melt

blending is a cost effective and less time consuming process

for the development of new materials with balanced prop-

erties [6, 7]. During melt blending, dipole interactions,

hydrogen bonding, or a combination of these occur naturally,

and such interactions can enhance the overall performance of

the resulting products [8, 9]. The modification can also be

made during processing to further improve the strength of the

material. Well established literature is available for modifi-

cation studies of polymer blends using techniques such as

R. Muthuraj � M. Misra � A. K. Mohanty

School of Engineering, Thornbrough Building, University of

Guelph, 50 Stone Road East, Guelph, ON N1G 2W1, Canada

R. Muthuraj � M. Misra (&) � A. K. Mohanty (&)

Department of Plant Agriculture, Bioproducts Discovery and

Development Centre, Crop Science Building, University of

Guelph, 50 Stone Road East, Guelph, ON N1G 2W1, Canada

e-mail: mmisra@uoguelph.ca

A. K. Mohanty

e-mail: mohanty@uoguelph.ca

123

J Polym Environ

DOI 10.1007/s10924-013-0636-5

in situ compatibilization [10], graft copolymerization [11],

copolymerization [12], and transreactions [8, 13]. Transre-

actions include an alcoholysis, acidolysis, and ester inter-

change reaction. These three reactions are generally referred

to as transesterification. The transesterfication reaction is an

exchange mechanism which can help to form a new type of

ester linkage between the components in the blends [14]. The

resulting transesterification products very often play

important roles in the miscibility, compatibility, crystallin-

ity, and mechanical properties of the blends. During the past

several years of research, many studies have been done on

the transesterification of polyester blends such as PBS/PCL

[6], poly(ethylene terephthalate)/poly(ether imide) [15],

poly(triethylene terephthalte)/polycarbonate [16], PHB/

PLA [17], and polycarbonate/poly(trimethylene terephthal-

ate) [18].

Among the biodegradable polyesters, both PBS and

PBAT have been intensively studied due to their inherent

biodegradability and commercial availability [1, 19]. PBS

is an aliphatic polyester which is synthesized from the

polycondensation reaction of petroleum based aliphatic

dicarboxylic acid (succinic acid) and 1,4-butane diol [20–

22]. The biodegradability of PBS is similar to cellulose and

bacterial polyesters like poly(hydroxybutyrate-co-valerate)

(PHBV) [23]. Currently, PBS is synthesized from renew-

able resource based succinic acid, which reduces its carbon

footprint while preserving its total performance [24]. PBS

is a good candidate for making biodegradable products as

well as having some unique physical properties such as

semicrystalline nature, thermal stability, good processing

properties, good gas barrier properties, and a lower melting

point [25–28].

Poly(butylene adipate-co-terephthalate) (PBAT) is com-

mercially synthesized from petroleum based adipic acid, 1,4-

butane diol, and terephthalic acid, which is a good biode-

gradable polymer in the presence of naturally occurring

microorganisms [29–31]. Furthermore, it has excellent

toughness and is mostly used for film extrusion and coatings

[32]. PBAT is a promising material to improve the toughness

of polymer blends which contain brittle polymers like

poly(lactic acid) [33], polycarbonate [34], and poly(-

hydroxybutyrate-co-valerate) [35]. As noted above, PBS and

PBAT are the most promising candidates for future biode-

gradable materials in various potential applications. Many

studies have reported the blending of either PBS or PBAT

with other biodegradable polymers. For instance, PBS has

been incorporated with many polymers such as poly(trieth-

ylene succinate) [21], poly(ethylene oxide) [36], poly(pro-

pylene carbonate) [37], poly(butylene terephthalate) [28],

copolyesters [30, 38], polyhydroxybutyrate [27], and

polycaprolactone (PCL) [6]. Although many studies have

reported stiffness-toughness balanced biodegradable binary

blends, to the best of our knowledge no literature is available

for PBS/PBAT binary blends. As two typical thermoplastic

biodegradable polyesters, the blend of PBS and PBAT are of

great interest due to their unique properties, which can

extend their applications in diversified areas. Hence, the

present work focused on the fabrication of a novel high

performance PBS/PBAT blend. The binary blend was pre-

pared by an extrusion followed by the injection molding

technique. The binary blends were characterized by differ-

ential scanning calorimetry (DSC), dynamic mechanical

analysis (DMA), polarizing optical microscopy (POM),

tensile properties, Fourier transform infrared spectroscopy

(FTIR), and scanning electron microscopy (SEM).

Experimental Section

Materials

Poly(butylene succinate) (PBS) pellets with a molecular

weight (Mw) of 1.4 9 105 g mol-1 and PDI of 1.82, com-

mercially named Bionolle 1020, manufactured by Showa

Highpolymer Co. Ltd, Japan, were used. The commercially

available PBAT (Biocosafe 2003F) was purchased from

Xinfu Pharmaceutical Co., Ltd, China. The molecular

structures of the neat polymers are shown in Scheme 1.

Blend Preparation

Prior to blending, polymer pellets were dried at 80 �C for

at least 12 h in a vacuum oven. Samples with different

compositions of PBS/PBAT were prepared in a micro

compounder (DSM Xplore, Netherlands). The micro

extruder was equipped with a co-rotating twin screw and

had a barrel volume of 15 cm3. A twin screw length of

150 mm and aspect ratio of 18 was used in the melt

blending process. The process temperature, cycle time, and

screw speed were kept constant at 140 �C, 2 min, and

100 rpm, respectively for different compositions of the

PBS/PBAT blend. The molten polymer was collected and

injected into the mold at 30 �C using a 12 cm3 micro-

Scheme 1 Molecular structure of PBAT and PBS

J Polym Environ

123

injection molder (DSM Xplore) kept at 140 �C. The mol-

ded test specimens were used for further characterization.

Fourier Transform Infrared Spectroscopy (FTIR)

FTIR analysis was performed by using a Thermo Scientific

Nicolet 6700 ATR-FTIR at ambient temperature. All the

scans were performed from 400 to 4,000 cm-1 with a

resolution of 4 cm-1. For each sample of the spectrum 32

accumulated scans were produced and the absorbance was

recorded as a function of wavenumbers.

Mechanical Properties

Tensile strength and percent elongation of the neat poly-

mers and their blends were obtained by Instron 3382, using

a constant strain rate of 50 mm min-1 at room tempera-

ture. The tensile testing was performed according to the

ASTM D638 testing method using dumbbell shaped sam-

ples. Data was collected by blue hill software. All the

reported values are an average of at least five samples for

each formulation.

Melt Flow Index (MFI)

Melt flow index of the neat polymers and their blends was

measured according to the ASTM D1238 standard using a

Melt Flow Indexer (Qualitest model 2000A) at 190 �C with

a standard weight of 2.16 kg. For each measurement, 6 g of

the material was loaded in the instrument. The presented

results are an average of at least five replicates of each

formulation.

Differential Scanning Calorimetry (DSC)

DSC analysis was carried out in a thermal analysis (TA)

instrument Q 200, and the analysis was performed under

the nitrogen atmosphere. Each sample, weighing about

5–10 mg, was placed in an aluminum pan and placed in the

instrument. The sample was first scanned from room tem-

perature to 150 �C with a heating rate of 10 �C min-1 and

subsequently cooled down from 150 to -50 �C at a cooling

rate of 5 �C min-1. A second heating scan of the samples

was performed from -50 to 150 �C at a rate of 10 �C

min-1. The first heating cycle was used for the removal of

thermal history and the reported results are from the second

heating scan. The data were analyzed using TA Instrument

Universal Analysis software.

Dynamic Mechanical Analysis (DMA)

The storage modulus and tan d of the neat polymers and

their blends were measured as a function of temperature by

a DMA Q800 from TA Instruments. The analysis was

performed between -50 and 100 �C at a heating rate of

3 �C min-1. The experiment was carried out in a dual

cantilever clamp with 1 Hz frequency and 15 lm oscil-

lating amplitude.

Heat Deflection Temperature (HDT)

HDT measurement was performed based on the ASTM

D648 standard at a constant load 0.455 MPa in the same

DMA instrument. The analysis was performed at a heating

rate of 2 �C min-1 from ambient temperature to 100 �C in

a three point bending mode.

Thermogravimetric Analysis (TGA)

Thermal stability of the neat polymers and their blends was

measured using a TA Q500 Instrument. Analysis was

performed under the nitrogen atmosphere at a flow rate of

60 ml min-1 from room temperature to 600 �C with a

heating rate of 20 �C min-1. The maximum rate of deg-

radation was observed from the derivative thermogram

(DTG).

Rheological Studies

A strain-controlled rheometer (Anton Paar Modular Com-

pact Rheometer MCR– 302) was used to observe the rhe-

ological properties of neat polymers and their blends.

Injection molded samples were placed between the paral-

lel-plates (diameter of the parallel plate is 25 mm), and the

experiment was performed at 140 �C using a gap width of

1 mm. Dynamic properties were determined by a dynamic

frequency sweep test. During the test, the range of fre-

quency was 500 to 0.01 rad s-1 and the strain was kept

constant at 3 % in the LVE region of neat polymers and

blends, respectively. These limits were fixed based on the

polymer torque sensitivity and their thermal stability.

Polarizing Optical Microscopy (POM)

Polarizing optical microscopy was performed on a Nikon,

Universal Design Microscope. The microscope was

equipped with a Linkam LTS 420 hot stage, which is used

to control the temperature. A DS-2Mv (with DS-U2) color

video camera with the capture NIS element imaging soft-

ware was used for POM observations. Samples were

sandwiched between two microscope glass slides and

heated to 150 �C for 5 min to remove the thermal history.

Subsequently, samples were annealed at the crystallization

temperature with a heating rate of 10 �C min-1. The

spherulities growth was observed at two different crystal-

lization temperatures of 80 and 90 �C.

J Polym Environ

123

Scanning Electron Microscopy (SEM)

Scanning electron microscopy images of the polymer

blends were captured by an Inspect S50—FEI Company.

The cryofractured samples were used to observe the phase

morphology of the blends. A selective dissolution of

polyester in tetrahydrofuran (THF) was used to distinguish

the polymer phases. All the samples were dried and sput-

tered with gold prior to imaging in order to make them

conductive.

Results and Discussions

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR analysis was performed to identify the physical

and chemical interaction between the neat polymers during

melt blending. The FTIR spectra of neat polymers and their

blends are shown in Fig. 1. The carbonyl group frequency

of neat polymers and their blends was observed at 1,712

and 1,716 cm-1, respectively. The peak of the carbonyl

group was shifted towards higher wavenumbers for the

PBS/PBAT blends compared to each of the neat polymers,

which clearly indicates that a strong chemical interaction

occurred during the melt process at 140 �C. Kwei [39] has

reported that the shift (from 1,722 to about 1,705 cm-1) of

the carbonyl group peak in the blends occurred as a result

of chemical interactions between the parent polymers. John

et al. [6] also observed a similar type of transesterification

reaction in the PBS/PCL and PCL/EASTAR blends. These

results can be explained by the formation of copolyester of

PBS and PBAT by an ester–ester interchange reaction. The

resulting copolyester which is compatible with the

homopolymer of the unreacted PBS and PBAT may play

the role of a compatibilizer in the blend system. In the

present study, no external transesterification catalyst was

added into the blends. Even though a transesterification

reaction was observed during melt blending, the amount of

the reaction gradually decreased with increasing PBAT

content. This resultant ester exchange reaction was due to

the presence of residual catalysts existing in the homo-

polymer synthesis [40–42]. The transesterification product

can further enhance the mechanical performance of the

resultant blends. Scheme 2 shows the expected chemical

structure of the transesterification product in the PBS/

PBAT blends.

Mechanical Properties

Figure 2 shows the stress–strain curves of PBS, PBAT, and

their blends. Neat PBS showed a higher tensile yield

strength but lower elongation compared to neat PBAT. For

PBS, no apparent strain hardening was observed during the

tensile test. On the other hand, PBAT showed excellent

elongation and obvious strain hardening regions in the

stress–strain curves, while its’ tensile and yield strength

was poor. As for the blends, all the samples presented three

clear regions such as elastic, plastic deformation, and strain

hardening. The first region showed linear stretching with

recoverable deformation, followed by the second region

which revealed plastic deformation which is a non recov-

erable deformation of the samples. The second region

indicated cold drawing behavior after the neck forming

occurred in the samples. The third region showed strain

hardening, the tensile stress gradually increasing until the

samples broke. Crystalline slippage was also observed.

Interestingly, after the strain increased to 150 %, each

composition of the blend showed clear evidence of cold

drawing which affected the polymer chain alignment and

resulted in strain hardening. Generally, amorphous and

semi crystalline polymer chain entanglement can lead to

strain hardening. Strain hardening behavior is of great

importance in polymer processing such as film blowing,

thermoforming, and blow molding due to its good resis-

tance against stretching of polymer segments. Also, strain

hardening behavior can make the process easier and lead to

higher quality products [43]. Figure 3 shows tensile

strength and percentage elongation data. A significant

improvement in the tensile strength and elongation was

observed for the blends.

The tensile strength of the blend was higher than that of

the neat polymers. The tensile strength of the PBS/PBAT

(70/30 wt %) blend increased by 30 and 148 % over the neat

PBS and PBAT, respectively. The percent elongation of

PBS/PBAT (70/30 wt%) blend was 150 % higher than neat

PBS. Tensile strength improvement is directly related to the

Fig. 1 Evaluation of the normalized FTIR spectra of the carbonyl

region (1,800–1,600 cm-1) of PBS, PBAT and their blends

J Polym Environ

123

intermolecular forces, crystallinity, miscibility, compatibil-

ity, and molecular orientation of the polymers in the blend

[6]. In the present study, tensile strength improvement was

directly related to the amount of PBS present in the blends.

Tensile strength decreased with increasing PBAT content in

the blend system, which reveals that the blend transesterifi-

cation ability was reduced. Furthermore, the increased

PBAT content in the blend system may cause phase sepa-

ration due to the decreased compatibility of the blend which

can lead to the reduction of tensile strength. These results

were also observed from morphological analysis of the

blends. John et al. [6] observed that a similar phase separa-

tion occurs when increasing one component in a PBS/

EASTAR and PCL/EASTAR blend leading to reduced ten-

sile strength in the blends. Tensile properties of the PBS/

PBAT blends were sharply increased compare to their parent

polymers. Apparently, the PBS/PBAT blends mechanical

properties are comparable with literature polyethylene

mechanical properties [44]. So, we believe that the PBS/

PBAT blends can be potential substitute for non-biode-

gradable polymers in the packaging applications.

Melt Flow Index

MFI measurement is a common technique for studying the

flow behavior of the polymers [45]. Table 1 shows the melt

flow index values of neat polymers and their blends. The MFI

value of PBAT and PBS was 9 and 25 g/10 min, respec-

tively. After blending both polymers, the MFI of all the

blends increased compared to the neat polymers. The

reduction of molecular weight and changing thermal prop-

erties during the melt blending may be responsible for this

[3]. PBS/PBAT (70/30 wt%) blend had the highest melt flow

rate compared to neat polymers and other PBS/PBAT blends.

The increased MFI of the blend can be related to a reduction

in molecular weight of PBS and PBAT through thermal

degradation because these polyesters are thermally sensitive.

Possible degradation can be occurring by chain scission of

the polymers, depolymerization, oxidative degradation, and

transesterification reactions. In addition, reactive end

groups, residual catalyst, unreacted starting monomers in the

Scheme 2 Expected

transesterification product of

PBS/PBAT blend

Fig. 2 Tensile stress–strain curves of PBS, PBAT, and their blends

Fig. 3 Tensile strength and elongation at break of PBS, PBAT, and

their blends: A PBS, B PBS/PBAT (70/30 wt%), C PBS/PBAT (60/

40 wt%), D PBS/PBAT (50/50 wt%), E PBAT

J Polym Environ

123

polymers, and other impurities can accelerate the thermal

degradation of the polymers [46]. Increased MFI of the

blends indicate better flow behavior compared to neat

polymers. Therefore, this blend system is suitable to use as a

new matrix system for polymer composites.

Differential Scanning Calorimetry

The thermal behaviors of the neat PBS, PBAT, and their

blends were measured through non-isothermal differential

scanning calorimetry analysis. Non-isothermal DSC results

of PBS, PBAT, and their blends are given in Fig. 4. The

neat PBS and PBAT showed melting points at 114 and

116 �C respectively. Interestingly, the second heating cycle

showed a double melting peak for neat PBS and their

blends due to the melt re-crystallization of the polymers.

During the second heating cycle as shown in Fig. 4, the

less perfect crystals melt at lower temperatures but the

more structurally perfect crystals melt at higher tempera-

ture [3]. Another possible reason may be that the molecular

weight distribution could also affect the melt of the poly-

mers [47]. The PBS/PBAT blend showed a similar melting

behavior to that of PBS. The enthalpy of fusion for neat

PBS and PBAT was found to be 32 and 10 J g-1, respec-

tively. The blends showed a high enthalpy of fusion com-

pared to the neat polymers. The PBAT phase may act as a

nucleating agent for the PBS phase, which will improve the

crystallization of PBS in the blend. Another reason is a

change in the regular structure of the interchange reaction

product, which may lead to the production of thicker

lamellar crystals that melt with a higher enthalpy of fusion

[6]. In all the blends, a single glass transition temperature

(Tg) was observed because the Tg values of both the neat

polymers are very close to each other. Therefore, the values

may be overlapping. The Tg value of the blends shifted to

lower temperatures compared with that of the neat poly-

mer. A similar observation was found through DMA ana-

lysis. This variation in Tg could be the cause of an

interchange reaction which occurs during the melt blending

process and it is also evidence for compatibility of the

polymer in the blends. John et al. [6] have observed similar

synergistic effects in PBS/PCL blends. Miscibility of the

two polymers can be predicted from Gordon-Taylor (G-T)

equation (Eq. 1) [48, 49]

Tg ¼W1Tg1 þ kW2Tg2

W1 þ kW2

ð1Þ

where Tg1 and W1 are the glass transition temperature and

weight fraction of PBAT, respectively. The Tg2 and W2 are

the glass transition temperature and weight fraction of PBS,

respectively, and the parameter k is the fitting constant. The

Tg values are observed from the DSC analysis. If k = 1,

the Gordon and Taylor theory represents a good interaction

between two blended components. If the k value is lower or

higher than 1, it indicates poor interaction between the

components in the blend. Figure 5 shows the Tg value

obtained from the G-T equation. From the DSC, the Tg

values observed for all the blends were -35 �C, close to

the G-T curve. This indicates good agreement with the G-T

equation. The k value of the present work was found to be

0.98; this semi-quantitatively measured value can further

support the interaction which occurred between the poly-

mers in the blends. The k value of the diglycidyl ether of

bisphenol-A/poly(ethylene terephthalate) blend was 0.10;

the small value of k suggests that only weak interactions

exist between the components in the blend [8, 50]. Richard

et al. [50] observed a k value of 0.18 for the PLA/PHBV

blend indicating poor miscibility, which was confirmed

through SEM analysis. These results are supportive of our

present work: that the components have a good compati-

bility in the blend system.

Miscibility and compatibility of the blends can be

explained by Gibbs free energy. Thermodynamically

compatible PBS/PBAT blends were analyzed according to

the Gibbs free energy equation (Eq. 2) [51]:

Table 1 Melt flow index (MFI) of the neat polymers and their blends

Samples MFI (g/10 min)

Neat PBS 25.3 ± 2.4

PBS/PBAT (70/30 wt%) 41.5 ± 3.2

PBS/PBAT (60/40 wt%) 33.3 ± 2.8

PBS/PBAT (50/50 wt%) 33.6 ± 1.6

Neat PBAT 9.4 ± 1.8

Fig. 4 Second heating DSC thermograms of PBS, PBAT, and their

blends after cooling at 5 �C/min

J Polym Environ

123

DGm ¼ DHm - T DScm þ DSe

m

� �ð2Þ

where DGm is Gibb’s free energy, T is absolute tempera-

ture, DHm is heat of mixing, DSem is mixing of combina-

torial entropy and DSem is the mixing of excess entropy. The

molar volume of the components inversely depends on the

combinatorial entropy. Hence, molecular weight of the

polymers is directly related to combinatorial entropy. If the

polymer has a higher molecular weight, the DScm becomes

zero. Therefore, the system is spontaneous; it could lead to

a DGm which is less than zero while DHm is less than zero.

In practical fields this is rarely possible and so can be

ignored. The DHm is calculated using expression Eq. (3)

[51]:

DHm ¼ ðd1�d2Þ2u1u2 ð3Þ

where d1, u1, d2, u2 are the solubility parameter values and

the volume fraction of PBS and PBAT. The solubility

parameter (d) of the PBS and PBAT was calculated as

follows (Eq. 4) [51]:

d ¼ qX

G� �

=M ð4Þ

where q, G and M are the density of the polymer, group

molar attraction constant of the polymer, and molecular

weight of the monomer unit, respectively. The group molar

attraction constant was calculated from Mark [52]. The

Gibbs free energy and solubility parameter values for PBS

and PBAT were calculated by Eqs. (3) and (4), and the

values are given in Table 2. Gibbs free energy values for

PBS/PBAT blends are low and very close to each other,

indicating that some extent of compatibility was achieved

in the blend system. Previous studies have reported similar

observations for some of the biopolymer blends such as

PLA/PCL, PLA/PHBV, and PHBV/PCL blends, and they

too have reported slight miscibility was achieved in their

melt blend process [7, 51, 53].

Dynamic Mechanical Analysis

Figure 6 shows the storage moduli of neat PBS, PBAT, and

their blends. The storage modulus value of the PBS and

PBAT at room temperature was found to be 0.6 and

0.1 GPa, respectively. PBS had a higher storage modulus

compared to PBAT at all temperatures, and their blends

had values in between the PBS and PBAT. A similar trend

was observed in the tensile modulus of the PBS/PBAT

blends. Reduction in modulus with increasing temperature

Table 2 Solubility parameter

values for polymersSample Group molar

attraction constant G

(J1/2 cm3/2 mol-1)

Solubility

parameter

d

(J1/2 cm3/2)

Gibbs free

energy DGm

(J g-1 m-3)

PBS 2,990 20.93 –

PBAT 6,154 22.28 –

PBS/PBAT (70/30 wt%) – – 0.382

PBS/PBAT (60/40 wt%) – – 0.437

PBS/PBAT (50/50 wt%) – – 0.455

Fig. 6 Storage moduli of PBS, PBAT, and their blends

Fig. 5 Theoretical and experimental values of Tg for PBS/PBAT

blends

J Polym Environ

123

is attributed to increasing polymer chain mobility. Gener-

ally, above the alpha transition temperature, molecular

motion increases and polymer segments move from glassy

to a rubbery state, which is accompanied by an increase in

the molecular relaxation in the polymers [54].

Figure 7 depicts the tan d curves of PBS, PBAT, and

their blends. It shows the primary and secondary transition

peaks in neat PBAT at -20 and 62 �C, respectively. The

primary transition peak corresponds to the poly(butylene

adipate) segment mobility, and the secondary transition

peak corresponds to the terephthalate unit mobility [55].

The Tg values of the neat polymers and their blends were

calculated from the maximum height of the tan d peak.

Generally, an incompatible blend shows two transition

peaks which correspond to the glass transition temperature

of individual components in the system [56]. For a highly

compatible and partially compatible blend, a single tran-

sition peak can be seen lying between the transition

temperature of individual components with an increased

broadness in the transition peak [9, 57]. In our present

study, all the blends were observed to have a single

transition peak. The Tg values of the PBS/PBAT blends

were shifted towards lower temperatures compared to the

neat PBS, which is a dilution effect with the addition of

PBAT into PBS. Another possible reason is that for par-

tially or completely compatible blends, the Tg shifts

towards lower or higher temperatures as a function of

composition and the broadness of the tan d peak [56].

Moreover, the small variation in the Tg value shows fur-

ther evidence of an interchange reaction occurring

between the neat homopolymers. The Tg shift was

observed by the influence of a transesterification reaction

when polycarbonate was incorporated into the poly(tri-

methylene terephthalate) [56].

Heat Deflection Temperature

Heat deflection temperature represents the maximum

working temperature of materials and is defined as the

temperature at which a material will be deformed by

250 lm under a constant load of 0.455 MPa [58]. The

HDT value of the neat polymers and their blends is shown

in Table 3. The HDT value of the neat PBS and PBAT is

88 and 46 �C, respectively. In general, the HDT of amor-

phous polymers is low, and around their glass transition

temperature. In the crystalline polymers, the HDT is close

to its melting point [7, 59]. In the present study, a balanced

HDT value of PBS/PBAT blends was observed due to the

PBAT having a lower crystallinity and thermal resistance

compared to PBS.

Thermogravimetric Analysis

Thermogravimetric analysis is the most accepted method

for studying the thermal stability of polymeric materials.

Figure 8 shows the thermal stability of PBS, PBAT, and

their blends as a function of temperature. PBS undergoes a

cyclic degradation mechanism and some of the predomi-

nant byproducts are anhydrides, olefins, carbon dioxide,

and esters [60]. PBAT degradation takes place by the

breakdown of the ester groups and chain scission of C–O

and C–C bonds on the polymer backbone. The onset deg-

radation temperature (Tonset) of PBAT was 377 �C and

PBS was 372 �C. This suggests that PBAT has slightly

more thermal stability compared to PBS. TGA results

reveal that PBS and PBAT present a relatively good ther-

mal stability up to 300 �C. These data are in good agree-

ment with the literature results [55, 61]. The maximum

degradation temperature (Tmax) was observed at 405, 413,

408, 410, and 413 �C for extruded PBS, PBAT, PBS/PBAT

(70:30 wt%), PBS/PBAT (60:40 wt%), and PBS/PBAT

(50:50 wt%), respectively. The Tonset and Tmax of the

blends were quite similar to those estimated for PBS and

PBAT homopolymer. All the blends showed single step

degradation because the neat polymer degradation tem-

peratures were close to each other, which was made clear

through a derivative thermogram (Fig. 9). According to

Fig. 7 Tan d curves of PBS, PBAT, and their blends

Table 3 Heat deflection temperatures of the neat polymers and their

blends

Samples HDT (�C)

Neat PBS 88.06 ± 0.4

PBS/PBAT (70/30 wt%) 73.66 ± 1.2

PBS/PBAT (60/40 wt%) 70.27 ± 2.1

PBS/PBAT (50/50 wt%) 65.88 ± 2.3

Neat PBAT 46.12 ± 1.5

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melt flow rate results, all the blends showed a molecular

weight reduction, but the thermal stability of the blends

was gradually increased compared to neat PBS. This may

be due to improved compatibility between the polymer

phases. While increasing the PBAT content, thermal sta-

bility increased because PBAT is more thermally stable

compared to PBS. The results indicate that a compatible

blend was achieved and supports the DSC results.

Rheological Properties

Rheological properties were investigated to identify the

interaction between polymer phases in the blends. Fig-

ure 10 shows the complex viscosity (g*) of the neat

polymers and their blends as a function of frequency at

140 �C. A higher complex viscosity was observed in the

lower frequency range compared to its higher frequency

region. Furthermore, the neat PBS and PBAT exhibited

almost Newtonian behavior at below 1 rad s-1 frequency,

and a strong shear thinning behavior was observed beyond

1 rad s-1 frequency range.

The PBS/PBAT blends had a higher complex viscosity

compared to the neat polymers at lower oscillation fre-

quencies. At higher frequencies, the complex viscosity of

the blends was between that of the neat polymers. This

increased viscosity may have occurred because transeste-

rification can form pseudo structures [62] that can with-

stand shear forces. The transesterification reaction also

plays a predominant role in viscosity improvement of the

blends. At lower frequency, the transesterification product

acts as a solid-like particle in all the PBS/PBAT blends and

leads to a higher viscosity compared to the parent poly-

mers. However, the FTIR results showed transesterification

product gradually decreased with increasing PBAT content

from 30 to 40 and 50 wt%. The higher content of PBAT

reduces the transesterification reaction in the blend system

and thus reduces the viscosity compared to PBS/PBAT

70/30 wt% blend. Li et al. [63] have reported similar

behavior for PLA/PBAT blends at lower frequencies and

also that the interaction between the polymers can increase

the melt viscosity. Also, this is due to phase morphology of

the blends and compatibility between the phases. The

higher compatibility between the two phases leads to good

dispersion of the discrete phase in the blend system. As we

can see in the SEM image, when PBAT content increases

from 30 to 50 wt% in the blend, the discrete phase (PBAT)

morphology is changed form spherical droplet to co-con-

tinuous morphology. This indicates that, the PBS/PBAT

70/30 wt% blend was more compatible than PBS/PBAT

60/40 and 50/50 wt% blends. Consequently, the PBS/

Fig. 8 TGA curves of PBS, PBAT, and their blends

Fig. 9 DTG curves of PBS, PBAT, and their blends

Fig. 10 Complex viscosity of PBS, PBAT, and their blends with

different weight fractions of PBAT at 140 �C

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123

PBAT 70/30 wt% blend had higher melt viscosity than

PBS/PBAT 60/40 and 50/50 wt% blend. A similar behav-

ior of the PLA/PBS and PLA/PBAT blends was reported in

literature [63, 64].

Figures 11 and 12 show the dynamic loss modulus and

the storage modulus of PBS, PBAT, and PBS/PBAT

blends. Generally, dynamic loss modulus and storage

modulus represent the amount of energy dissipated in the

viscous portion and the ability of a material to store energy

during deformation, respectively. Figure 11 shows that the

storage modulus (G0) of each sample increased with

increase in frequency. It was also observed that with the

blending of PBAT into PBS, there were no changes in the

storage modulus (G0) at higher frequencies. All the blends

showed a higher storage modulus at lower frequencies

compared to the neat polymers. The 70:30 wt% of PBS/

PBAT blend had a higher loss modulus (G00) than other

blends (Fig. 12). When the PBAT phase was finely dis-

persed in the blend, the fine dispersal could be the reason

for interaction existing between the two phases. Stronger

interactions were observed in the PBS/PBAT (70:30 wt%)

blend, which were exhibited as a higher loss modulus. In

addition, the increased storage modulus is attributed to the

PBAT molecular chain entanglement with PBS molecular

chain mediated by the transesterification product acting as

a compatibilizer. The higher entanglement density of the

blends would store more recoverable energy. Generally,

the entanglement density of the blend depends on the

existing compatibility between the two phases. The PBS/

PBAT 70/30 wt% blend had more compatibility than PBS/

PBAT 60/40 and 50/50 wt% blends as shown in SEM. The

higher entanglement density of the PBS/PBAT 70/30 wt%

blend leads to higher storage modulus. However, the

storage modulus gradually decreased at lower frequency

with increasing PBAT content in the blends. A similar

trend was observed in the complex viscosity. This is con-

sistent with a higher amount of trasesterification product

present in 70/30 wt% blend. The reduced storage modulus

of the blends is due to the morphology changes [63] and

entanglement density due to lower transesterification

product present in the blend.

The Cole–Cole plot was used to explain the phase

structure of PBS/PBAT blends and the plot was performed

between the real and imaginary viscosity components of

the blends. If the blend gives a single arc curve, it can

suggest phase homogeneity at the melt stage [65]. Fur-

thermore, if any deviations from a single arc are observed,

they are evidence for inhomogeneous morphology and

phase separation occurring in the blends due to a second

relaxation mechanism occurring in the samples. The Cole–

Cole plot for PBS/PBAT blend at 140 �C is depicted in

Fig. 13. A second circular arc was observed on the right-

hand side of the curve, and it is clear evidence for a second

Fig. 11 Loss modulus versus frequency for PBS, PBAT, and their

blends with different weight fractions of PBAT at 140 �CFig. 12 Storage modulus versus frequency for PBS, PBAT, and their

blends with different weight fractions of PBAT at 140 �C

Fig. 13 Cole–Cole plot of the PBS/PBAT blends at 140 �C

J Polym Environ

123

Fig. 14 (a) Photograph of the

film annealed at 80 �C: (i) PBS;

(ii) PBS/PBAT (70/30 wt%);

(iii) PBS/PBAT (60/40 wt%)

and (iv) PBS/PBAT (50/

50 wt%). (b) Photograph of the

film annealed at 90 �C: (i) PBS;

(ii) PBS/PBAT (70/30 wt%);

(iii) PBS/PBAT (60/40 wt%)

and (iv) PBS/PBAT (50/

50 wt%)

J Polym Environ

123

relaxation mechanism happening for all PBS/PBAT blends.

Nevertheless, when the PBAT reaches 40 and 50 wt% in

the blends, the blends showed a tail on the right hand side

of the plot. This is probably due to the phase inversion

occurring in the blends. This result shows that co-existing

phase morphology was formed in the entire blend system

Fig. 15 (a) SEM images of PBS and PBAT blends (left hand side):

(i) PBS/PBAT (70/30 wt%); (ii) PBS/PBAT (60/40 wt%) and (iii)

PBS/PBAT (50/50 wt%). (b) SEM images of PBS and PBAT blends

surface after etching with THF (right hand side): (i) PBS/PBAT (70/

30 wt%); (ii) PBS/PBAT (60/40 wt%) and (iii) PBS/PBAT (50/

50 wt%)

J Polym Environ

123

and also that the formed morphology may be the droplet-

matrix or co-continuous phase morphology. Consequently,

the Cole–Cole plot shows an inhomogeneous morphology

formed in PBS/PBAT blend systems. There have been

reports of PLA/PBAT blend systems with similar obser-

vations of phase behavior when the PBAT concentration is

more than 30 wt% in the matrix [63]. The Cole–Cole plot

clearly shows that the PBS/PBAT 70/30 wt% blend is more

heterogeneous compared to the 50/50 wt% blend.

Polarizing Optical Microscopy

The crystallization behavior of PBS and PBS/PBAT blends

was investigated by optical microscopy. The dark and light

regions represent the amorphous and crystalline phases,

respectively. Figure 14a shows that PBS/PBAT blends were

annealed at a crystallization temperature of 80 �C for

30 min. When the PBAT content was 50 wt% in the blend,

an increased number of PBS spherulites were observed with

decreasing spherulite size. This decrease in size suggests that

the PBS chain mobility was disrupted and PBAT acted as a

nucleating site to promote the formation of crystalline nuclei.

A PBS/PVDF blend exhibited a similar phenomenon when

PVDF was the predominant species in the blend compared to

the PBS [66]. The PBS chain mobility was slowed in the

presence of highly viscous PBAT, thereby decreasing

spherulite growth. With increasing PBAT content in the

blends (from 30 to 50 wt%), the texture of PBS spherulites

became coarse. Figure 14b shows the PBS/PBAT blends

after being annealed at a crystallization temperature of 90 �C

for 30 min. The increasing PBAT composition in the blends

caused a reduced number of spherulites because PBAT has

less crystallinity and the PBS chain mobility increases at the

crystallization temperature of 90 �C compared to 80 �C.

With increasing PBAT content in the blends, the spherulites

became rougher. For 30 wt% PBAT content in the blend, the

PBS spherulites were uniformly distributed with uniform

dimensions after being annealed for a given time. Conse-

quently, our present results conclude that the spherulite size

plays a vital role in the control of morphology and

mechanical properties of the blends.

Scanning Electron Microscopy

For the polymer blends, phase morphology formed in the

blend plays a vital role in determining the properties of the

resulting blend, such as mechanical and thermal properties

as well as permeability. Phase morphology of the blends

depends on the second components, processing parameters,

molecular weight of the virgin polymers, and compatibility

between the polymers. If the blending components have a

similar melt viscosity, the resulting morphology will be

very fine and both polymers will be uniformly distributed

throughout the blend whether it is the minor or major

phase. The same is true if the blend consists of similar melt

viscosity components. If the minor phase has a lower or

higher viscosity compared to the major phase, it leads to

the spherical domains of finely or coarsely dispersed

morphology in the matrix. As shown in Fig. 15, phase

morphology in the blend was identified by the solvent

etching method. Being a good solvent for PBAT while

unable to dissolve PBS, THF was used as the etching

solvent. The observed morphology of PBAT phase selec-

tively removed from the blends without disturbing the PBS

matrix is shown in Fig. 15b. The PBAT phase was com-

pletely extracted under these conditions. The correspond-

ing unextracted samples are shown in Fig. 15a. This

indicates that the holes are the extracted PBAT phase by

THF. The surface morphology of the blend reveals that

spherical PBAT particles were uniformly distributed

throughout the matrix. Finer dispersions were observed in

lower PBAT composition in the blend. When the PBAT

composition was increased in the blend, the domain shape

and size gradually changed due to the coalescence phe-

nomenon. This may also act to decrease the tensile strength

of the blends.

Conclusions

We have succeeded in fabricating a high performance and

biodegradable PBS/PBAT blend through the melt blending

technique. There is a significant improvement in tensile

strength and elongation at break by the incorporation of

PBAT into PBS, indicating that a good level of compati-

bility is achieved between the polymers. The observed

compatibility is caused by the formation of copolyester due

to transesterification between the neat polymers, which was

confirmed by FTIR analysis. DSC and DMA analysis

suggested that the blends show compatibility between the

PBS and PBAT. The rheological properties of blends such

as the complex viscosity, storage modulus, and loss mod-

ulus were increased with the addition of PBAT in the

matrix. As the PBAT composition was increased, phase

morphology changes occurred in the blends, leading to

decreased values of complex viscosity, storage modulus,

and loss modulus. The phase morphology of the PBS/

PBAT blends shows a two phase structure in which PBAT

is the minor phase. Furthermore, polarizing optical

microscopy analysis revealed that the PBAT has disturbed

the spherulite growth of the matrix.

Acknowledgments The authors acknowledge the Ontario Ministry

of Agriculture and Food (OMAF) and Ministry of Rural Affairs

(MRA)—University of Guelph Bioeconomy-industrial uses research,

for their sponsorships. They also gratefully acknowledge the Ontario

Research Fund, Research Excellence, round-4 (ORF RE04) from

J Polym Environ

123

Ontario Ministry of Economic Development and Innovation (MEDI),

Natural Sciences and Engineering Research Council (NSERC), Net-

works of Centers of Excellence (NCE) and AUTO21 project for their

financial support to carry out this research work.

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